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Level 3 Electrical, BCIT > Three Phase Transformers > Flashcards

Three Phase Transformers Flashcards

1
Q

What is an isolation transformer?

A

Zero electrical or physical connection between the primary and secondary windings, works off of mutual induction

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2
Q

What are the four different application ratings?

A

Power Supply (over 500kVA)
Distribution (500kVA and under)
Control (Steps down to 120v)
Instrument (CT’s and PT’s)

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3
Q

Why is the isolation transformer most common?

A

It is the best for controlling and limiting current spikes

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4
Q

What are the properties of the high voltage winding?

A

More turns, thinner wire, thicker insulation

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5
Q

What are the properties of the low voltage winding?

A

Fewer turns, larger wire, thinner insulation

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6
Q

What are the properties of the transformer core?

A

Soft iron, high permeability, low retentivity, high resistance

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7
Q

When connection leads are on the same on the left hand side (H1, X1) the windings are additive or subtractive?

A

Subtractive.

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8
Q

When connection leads are different on the left hand side (H1, X2) the windings are additive or subtractive?

A

Additive

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9
Q

What is ‘excitation current’ or ‘magnetizing current’

A

When there is no load on the secondary, the primary winding will ‘self induce’ a current into the secondary winding. Usually 2-5% of the primary current.

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10
Q

What are the three rules for connected transformers in parallel?

A

Must have the same impedance, connected polarities must match, and same voltage ratings

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11
Q

T or F: transformers must be sized for the largest rated coil?

A

True

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12
Q

How do you find efficiency of a transformer?

A

Power out / power out + losses

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13
Q

How do you find fault current?

A

Rated amps of secondary / %Z

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14
Q

Which test will find copper losses?

A

Short circuit test

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15
Q

Which test will find core or iron losses?

A

Open circuit test

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16
Q

What is a short circuit test?

A

Full load current is measured to calculate copper losses. Current losses increase as load increases because copper losses are directly proportional to magnitude.

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17
Q

What is voltage regulation?

A

The change in voltage from no load to full load.

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18
Q

How do you find fault current?

A

I rated / impedence

I rated is full load amps of the secondary

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19
Q

Impedance voltage is usually ___% of rated voltage?

A

3-5%

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20
Q

Impedance voltage is also called ____

A

Short circuit voltage.

%Z = E short circuit / E rated

21
Q

What are the three advantages of three phase TX’s?

A
  1. Cheaper equipment and installation
  2. Smaller equipment, weighs less and easier to install
  3. More efficient
22
Q

When the polarity of a wye connected secondary is reversed, what happens to the line voltage?

A

It will read phase voltage (which is root 3 less)

23
Q

On a dual winding TX, how do you find the transformation ratio?

A

Use only one set of windings.
Ex) primary winding is connected in series and each winding is seeing 240V, 480V in total for the TX. The secondary winding is connected in parallel, each winding is seeing 208V, 208V in total for the TX.

Use 240V / 208V to find the ratio, do not use 480V / 208V

24
Q

Which connection will all X1’s be neutral connected?

A

Delta to Wye step down, or Wye to Delta step up

25
Q

What is the difference between an open delta and a closed delta circuit?

A

Closed delta: all tx’s are connected
Open delta: only two tx’s are connected

26
Q

When connecting an open delta, what test must be done?

A

Voltage test. not the same as delta closure test

27
Q

In an open delta connection, when performing the voltage test, what would the voltage read if the connection was done improperly?

A

Root 3 of the line voltage

28
Q

When performing a delta closure test, what would the voltmeter read if the connections were improper?

A

Double the line voltage

29
Q

When performing a delta closure test, what would the voltmeter read if the connections were done correctly?

A

0 - line voltage.

30
Q

Potential TX’s have a standard output voltage of ____

A

120V

31
Q

Current TX’s have a standard output of ____A

A

5A

32
Q

What is the danger presented by CT’s?

A

Opening the secondary of a CT will cause the secondary voltage to rise to dangerous levels

33
Q

How do you find the wattmeter ratio?

A

PT ratio multiplied by the CT ratio

34
Q

What is the phase displacement between a wye-wye or delta-delta TX bank?

A

There is no phase displacement. Van is at the same angle on both the high and low side

35
Q

What is the most common / practical TX system?

A

3 phase 4 wire Delta 120/240V

36
Q

Which winding is the standard to be centre tapped?

A

B

37
Q

In a Delta - Wye step down or a Wye - Delta step up, which point is connected to neutral?

A

All X1’s

38
Q

What are some disadvantages to an auto transformer?

A

Lower impedance resulting in higher fault current
High transformation ratios causing shock hazards and damage to equipment

39
Q

Can auto transformers be used as a distribution TX in a building?

A

No. Can only be used for one piece of equipment

40
Q

What are the limited transformation ratios of an auto transformer?

A

2:1 and 3:1

41
Q

What are the practical applications for an auto transformer?

A

Bucking and boosting TX’s for a specific piece of equipment (most common)
As a reduced voltage starter for a motor

42
Q

Delta auto transformers are restricted to which ratio?

A

2:1

43
Q

What is power quality issues?

A

Anything that distorts the sine wave

44
Q

What are brown outs?

A

Sustained low voltage. Burns out equipment.

45
Q

What are harmonics?

A

Non linear loads (something that switches) waveforms that are multipliers of the fundamental frequency.

Ex) second harmonic is 120Hz, third is 180hz

46
Q

What is a Triplen?

A

Anything that is multiplied by 3

47
Q

What is the problem with Triplens?

A

Cause O/H of neutrals.

48
Q

Which code rule outlines sizing of the neutral and grounding conductors?

A

4-032

Level 3 Electrical, BCIT (7 decks)

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